Physical Chemistry Chemical Physics vol:3 issue:15 pages:3070-3074
The absolute rate coefficient of the title reaction was measured for the first time at elevated temperatures, using a pulsed laser photolysis/chemiluminescence (PLP/CL) technique. C2H radicals were generated by excimer laser photodissociation of acetylene at 193 nm, and pseudo-first-order decays of thermalised C2H were monitored in real-time using the CH(A(2)Delta -->X(2)Pi) chemiluminescence from the reaction of C2H with excess O-2 as an ethynyl probe. The rate coefficients, measured over the temperature range 295-779 K, exhibit a slight non-Arrhenius behaviour and can be represented by k(C2H+C2H6)(T)=1.19x10(-12) T0.54 +/-0.20 exp[(180 +/- 70) K/T] cm(3) molecule(-1) s(-1). Ab initio calculations were performed at the MP2 and CCSD(T) levels of theory to elucidate the molecular mechanism of the C2H+alkane reactions. It is concluded that direct H-abstraction, leading to C2H2+alkyl radical, has by far the lowest energy barrier, approximate to3 kJ mol(-1), whereas substitution and insertion reactions face barriers in excess of 100 kJ mol(-1) and are therefore not competitive.